Substrate for surface acoustic wave device and surface acoustic wave device

ABSTRACT

An SAW device substrate and an SAW device having a larger value of K 2  than conventional SAW device substrates can be obtained by using LiNbO 3  represented by Eulerian angles of (18-30°, 80-100°, 35-75°) and determining the thickness H of a piezoelectric substrate and the pitch λ of electrodes so that KH is at least 2.3 and at most 4.5. Alternatively, an Li 2 B 4 O 7  layer is formed as a piezoelectric substrate on a surface of a glass layer to obtain an Li 2 B 4 O 7 /glass structured SAW device substrate. More preferably, such Li 2 B 4 O 7  that is represented by Eulerian angles of (0-45°, 85-95°, 85-95°) is employed. Furthermore, the SAW device substrate is used to form an SAW device so that the KH parameter is about 0.5.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface acoustic wave (SAW) device.

2. Description of the Background Art

For example, the SAW devices include an SAW resonator, a transversaltype SAW filter and a resonator type SAW filter which are selectivelyused according to application. In the following, an SAW filter, forexample, as one of the SAW devices will be described.

FIG. 28 is a perspective view schematically showing the structure of anSAW filter. Referring to FIG. 28, the basic structure of the SAW filter15 is a four-terminal structure which has a piezoelectric substrate 12and a pair of comb-shaped electrodes 13 for surface wave excitation andreception formed on a surface of piezoelectric substrate 12. This typeof electrode 13 is called an interdigital electrode, and this type of atransduction element is called an IDT (interdigital transducer).

In general, when an impulse voltage is applied to comb-shaped electrode13 for oscillation, the piezoelectric effect causes opposite phasestrain between adjacent electrodes 13 as shown in FIG. 29, and an SAW isexcited. The SAW propagates on the surface of piezoelectric substrate12. A surface electric charge is caused on piezoelectric substrate 12 bythe strain, which is brought about by the surface wave, and extracted asan electric signal by comb-shaped electrode 13 for reception.

Conventionally, an SAW device such as SAW filter 15 has a structure inwhich an electrode corresponding to each device is arranged on thesurface of piezoelectric substrate 12 as shown in FIG. 30. Thecharacteristics of SAW device 15 largely depend on those ofpiezoelectric substrate 12, and piezoelectric substrate 12 is alsoselectively used according to application. Table 1 indicates typicalmaterials for piezoelectric substrate 12 and the characteristics of anSAW which piezoelectric substrate 12.

TABLE 1 Various Characteristics of Typical SAW Device Substratespropaga- Eulerian tion TCD angles velocity K² [ppm/ propagationsubstrate φ, θ, ψ [m/s] [%] ° C.] mode crystal 0°, 132.75°, 0° 3159 0.120 Rayleigh wave 0°, 15°, 0° 3948 0.11 0 leaky wave LiTaO₃ 90°, 90°, 112°3328 1.0 23.3 Rayleigh wave 0°, 126°, 0° 4211 4.7 45.1 leaky waveLi₂B₄O₇ 45°, 90°, 90° 3465 0.80 −8.2 Rayleigh wave 0°, 75°, 75° 4120 1.6−1.5 leaky wave LiNbO₃ 0°, 38°, 0° 4007 5.2 71.4 Rayleigh wave 0°, 154°,0° 4731 10.9 61.3 leaky wave

As can be seen from Table 1, the crystal substrate has low and superiortemperature characteristics but it has small electro-mechanical couplingcoefficients (K²). Conventionally, a 128° Y-X LN substrate (expressed byEulerian angles of (0°, 38°, 0°) LN) is generally used for an LiNbO₃(LN) substrate. However, the conventional LN substrate has large valuesof K² but it has inferior temperature characteristics such as atemperature coefficient of delaytime (TCD). On the other hand, theLi₂B₄O₇ substrate has intermediate characteristics between those of thecrystal substrate and the LN substrate.

As described above, each substrate has both advantageous anddisadvantageous points and is selectively used according to applicationof the device. In recent years, video devices such as a TV andtelecommunication devices such as a mobile phone have been developed,and therefore SAW devices used therefor need to have characteristicssuperior than ever.

Here, the Eulerian angles in Table 1 will be described with reference toFIG. 31.

Referring to FIG. 31, the Z axis is first used as a rotational axis torotate the X axis in the Y axis direction by φ so as to provide a firstaxis. Then, the first axis is used as a rotational axis to rotate the Zaxis counterclockwise by θ so as to provide a second axis. A substrateis obtained by cutting a substrate material at a plane orientation suchthat the second axis is normal to the plane and the first axis is on theplane. In the substrate which is cut at the orientation, the second axisis used as a rotational axis to rotate the first axis counterclockwiseby ψ to provide a third axis. The third axis is employed as an SAWpropagation direction. It is noted that an axis perpendicular to thethird axis on the plane is a fourth axis. In this manner, the Eulerianangles (φ, θ, ψ) are defined.

Recently, attaining smaller SAW devices have been requested. It isconsidered in general that a device with a large bandwidth can bedesigned more easily as the value of K² of a piezoelectric substrate ishigher. Furthermore, a substrate with a large value of K² makes itpossible to reduce the number of electrodes, and it is advantageous forproviding a smaller SAW device. Conventionally, however, K² is a valueinherent in a substrate material although it more or less increases ordecreases with Eulerian angles employed. Conventionally, it is thereforenecessary to begin with the development of a substrate material in orderto obtain an SAW device substrate having a large value of K². If thiscan be achieved by using conventional materials, that can greatlycontribute to the technical field.

Here, the center frequency f_(O) of an SAW device is determined byf_(O)=V/λ (V: the propagation velocity of a surface acoustic wave, λ:the electrode pitch of an IDT). Therefore, if the Li₂B₄O₇ substratehaving a higher propagation velocity V than that of a crystal substrateor the like is simply used for manufacturing a device having the samecenter frequency f_(O), the electrode pitch λ of the IDT needs to beincreased and, as a result, the SAW device itself has to be made larger.In short, a substrate with a lower propagation velocity V isadvantageous for attaining a smaller SAW device itself.

Therefore, the present invention aims to attaining a smaller SAW deviceas a common object. For the purpose, the present invention has a firstobject of obtaining, by using a conventional material, an SAW devicesubstrate which has larger K² advantageous for attaining a smaller andhigher performance SAW device, and a second object of obtaining an SAWdevice substrate which has intermediate K² between those of a crystalsubstrate and an LN substrate and has a low propagation velocityadvantageous for attaining a smaller SAW device.

SUMMARY OF THE INVENTION

The inventors conducted an extensive study based on the above objectsand found out that larger K² can be obtained by setting Eulerian anglesand the value of KH in prescribed ranges, wherein K is a value found bydividing 2π by an electrode pitch, H is a piezoelectric substratethickness, and KH is a product of K and H. In the present invention, theabove object is attained by forming LN, which is expressed as (18-30°,80-100°, 35-75°) in terms of Eulerian angles, on a glass surface so asto provide a (18-30°, 80-100°, 35-75°) LN/glass structure substrate.

In the (18-30°, 80-100°, 35-75°) LN/glass structure substrate, it ismore preferable that much higher K² can be obtained by setting a KHparameter so that 2.8≦KH≦3.8.

In the present invention, the above described objects are also achievedby forming an Li₂B₄O₇ layer on a glass surface to obtain anLi₂B₄O₇/glass structure substrate. More preferably, such Li₂B₄O₇ that isexpressed as (0-45°, 85-95°, 85-95°) in terms of Eulerian angles is usedin the Li₂B₄O₇/glass structure substrate. More preferably, the KHparameter is set at about 0.5 to obtain an SAW device substrate having alower propagation velocity.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing the structure of an SAWdevice in an embodiment of the present invention.

FIG. 2 illustrates the dependence of K² of a (30°, 90°, 60°) LN/glassstructure substrate on KH.

FIG. 3 is a graph illustrating the calculation result of K² of a (0°, θ,ψ) LN/glass structure substrate for KH which provides the maximum K².

FIG. 4 is a graph illustrating the calculation result of K² of a (10°,θ, ψ) LN/glass structure substrate for KH which provides the maximum K².

FIG. 5 is a graph illustrating the calculation result of K² of a (20°,θ, ψ) LN/glass structure substrate for KH which provides the maximum K².

FIG. 6 is a graph illustrating the calculation result of K² of a (30°,θ, ψ) LN/glass structure substrate for KH which provides the maximum K².

FIG. 7 is a graph illustrating the calculation result of K² of a (φ,90°, 60°) LN/glass structure substrate for KH which provides the maximumK².

FIG. 8 illustrates the dependence of K² of a (30°, 90°, 60°) LN/glassstructure substrate on KH when 2.3≦KH≦4.5.

FIG. 9 is a graph illustrating the value of KH which causes K² of a (0°,θ, ψ) Li₂B₄O₇/glass structure substrate to be 0.80%.

FIG. 10 is a graph illustrating the propagation velocity (m/s) of the(0°, θ, ψ) Li₂B₄O₇/glass structure substrate for the KH shown in FIG. 9.

FIG. 11 is a graph illustrating the TCD (ppm/° C.) of the (0°, θ, ψ)Li₂B₄O₇/glass structure substrate for the KH shown in FIG. 9.

FIG. 12 is a graph illustrating the value of KH which causes K² of a(10°, θ, ψ) Li₂B₄O₇/glass structure substrate to be 0.80%.

FIG. 13 is a graph illustrating the propagation velocity (m/s) of the(10°, θ, ψ) Li₂B₄O₇/glass structure substrate for the KH shown in FIG.12.

FIG. 14 is a graph illustrating the TCD (ppm/° C.) of the (10°, θ, ψ)Li₂B₄O₇/glass structure substrate for the KH shown in FIG. 12.

FIG. 15 is a graph illustrating the value of KH which causes K² of a(20°, θ, ψ) Li₂B₄O₇/glass structure substrate to be 0.80%.

FIG. 16 is a graph illustrating the propagation velocity (m/s) of the(20°, θ, ψ) Li₂B₄O₇/glass structure substrate for the KH shown in FIG.15.

FIG. 17 is a graph illustrating the TCD (ppm/° C.) of the (20°, θ, ψ)Li₂B₄O₇/glass structure substrate for the KH shown in FIG. 15.

FIG. 18 is a graph illustrating the value of KH which causes K² of a(30°, θ, ψ) Li₂B₄O₇/glass structure substrate to be 0.80%.

FIG. 19 is a graph illustrating the propagation velocity (m/s) of the(30°, θ, ψ) Li₂B₄O₇/glass structure substrate for the KH shown in FIG.18.

FIG. 20 is a graph illustrating the TCD (ppm/° C.) of the (30°, θ, ψ)Li₂B₄O₇/glass structure substrate for the KH shown in FIG. 18.

FIG. 21 is a graph illustrating the value of KH which causes K² of a(40°, θ, ψ) Li₂B₄O₇/glass structure substrate to be 0.80%.

FIG. 22 is a graph illustrating the propagation velocity (m/s) of the(40°, θ, ψ) Li₂B₄O₇/glass structure substrate for the KH shown in FIG.21.

FIG. 23 is a graph illustrating the TCD (ppm/° C.) of the (40°, θ, ψ)Li₂B₄O₇/glass structure substrate for the KH shown in FIG. 21.

FIG. 24 is a graph illustrating the value of KH which causes K² of a(45°, θ, ψ) Li₂B₄O₇/glass structure substrate to be 0.80%.

FIG. 25 is a graph illustrating the propagation velocity (m/s) of the(45°, θ, ψ) Li₂B₄O₇/glass structure substrate for the KH shown in FIG.24.

FIG. 26 is a graph illustrating the TCD (ppm/° C.) of the (45°, θ, ψ)Li₂B₄O₇/glass structure substrate for the KH shown in FIG. 24.

FIG. 27 is a graph illustrating the frequency characteristics of SAWfilters using a <110> Li₂B₄O₇ substrate and an Li₂B₄O₇/glass structuresubstrate.

FIG. 28 is a perspective view showing the structure of a general SAWfilter according to the conventional art.

FIG. 29 is a view for illustrating an operation of the SAW filter shownin FIG. 28.

FIG. 30 is a sectional view schematically showing the structure of anSAW device according to the conventional art.

FIG. 31 is a view for illustrating Eulerian angles.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the present invention will be describedwith reference to the drawings.

First Embodiment

A schematic sectional view showing the structure of an SAW device in afirst embodiment of the present invention is shown in FIG. 1. An SAWdevice 5 has a glass substrate 1, a piezoelectric substrate 2 and anelectrode 3. Piezoelectric substrate 2 is formed on glass substrate 1and is made of LN. On piezoelectric substrate 2 is formed electrode 3which has an arrangement according to the device. It is noted that aportion formed of glass substrate 1 and piezoelectric substrate 2 iscalled a substrate for the SAW device (hereinafter, referred to as anSAW device substrate) 6.

Such an SAW device 5 can be obtained by bonding an LN substrate aspiezoelectric substrate 2 onto a surface of glass substrate 1 or formingan LN thin film as piezoelectric substrate 2 on the surface of glasssubstrate 1 and, thereafter, forming and patterning electrode 3 on theLN substrate or the LN thin film.

Glass substrate 1 may be made of a material such as quartz glass,aluminosilicate glass, borosilicate glass, soda lime glass and leadsilicate glass. The substrate to be combined with the LN substrate isnot limited to a glass substrate, and it may be any material causing alower sound speed than LN and having an opposite sign characteristicfrom those of TCD and TCV (temperature coefficients of sound speed) ofLN.

Piezoelectric substrate 2 may be made only of LN or may partiallycontain LN. The material for electrode 3 may be aluminum, for example,although not exclusive.

As shown in FIG. 1, the thickness of piezoelectric substrate 2 is H, thepitch of electrodes 3 is λ, and K is 2π/λ. The following descriptionwill be provided by paying attention to the Eulerian angles of LNincluded in piezoelectric substrate 2 as well as KH which is a productof K and H.

FIG. 2 illustrates the result of calculating K² in a (30°, 90°, 60°)LN/glass structure substrate. The calculation is performed by using KHas a parameter and employing the Campbell and Jones method. As can beseen from FIG. 2, the value of K² depends on the value of KH. It is alsofound out that K² has the maximum value when KH is 3.3.

Here, a parameter φ of Eulerian angle parameters (φ, θ, ψ) whichindicate an orientation is varied to the four values of 0°, 10°, 20°,30° and θ and ψ are changed from 0° to 180° in each case. By a similarcalculation, KH which provides the maximum value of K² in eachorientation (φ, θ, ψ) is found, and the maximum value of K² is obtainedby using the found value of KH. The calculation result is illustrated inFIGS. 3 to 6. In FIG. 3, φ is fixed at 0° and θ and ψ are changed. InFIG. 4, φ is fixed at 10° and θ and ψ are changed. In FIG. 5, φ is fixedat 20° and θ and ψ are changed. In FIG. 6, φ is fixed at 30° and θ and ψare changed. In these figures, the maximum value of K² in eachorientation is represented in contour. A number with a contour line isthe maximum value of K² in its orientation.

Since the object of the present invention is to obtain an SAW devicesubstrate having a larger value of K² than a conventional (0°, 38°, 0°)LN substrate, it is less significant to evaluate orientations which canattain only K² smaller than 5.2% that is the value of K² for theconventional substrate. Therefore, orientations which attain the valuesof K² smaller than 5% are not shown in FIGS. 3 to 6. In other words,portions with no contour lines correspond to the orientations whichattain the values of K² smaller than 5% in FIGS. 3 to 6.

According to the calculation result, the maximum value of K² is as largeas over 8% when the Eulerian angles of LN are in the range of (20°,80-90°, 50-70°) in FIG. 5 and in the range of (30°, 80-100°, 35-75°) inFIG. 6. Above all, when the range is (30°, 90°, 60°), the maximum valueof K² is as large as 8.7%.

FIG. 7 illustrates the result of a similar calculation in the range of(0-30°, 90°, 60°). It can be seen from FIG. 7 that the maximum value ofK² continuously changes in the range in which φ is 0-30°. Therefore, themaximum value of K² for φ other than when φ=0°, 10°, 20°, 30° can alsobe predicted because of the continuity. Furthermore, the value of K² canbe a large value close to 8.7% even if the range is not (0-30°, 90°,60°) but it is (about 0-30°, about 90°, about 60°).

It can be seen from FIG. 7 that the maximum values of K² exceeds 8.0% inthe range of 18°≦φ. Therefore, the Eulerian representation is preferably(18-30°, 80-100°, 35-75°).

From FIG. 7, the maximum value of K² is as large as over 8.5% in therange of 24°≦φ. Meanwhile, it can be seen from FIG. 6 that under thecondition of φ=30° the maximum value of K² is largest in the ranges of88°≦θ≦92° and 55°≦ψ≦65°. Therefore, the Eulerian representation ispreferably (24-30°, 88-92°, 55-65°).

FIG. 8 illustrates the value of K², calculated in the range of 2.3≦KH≦4.5, when a (30°, 90°, 60°) LN is used. It can be seen from FIG. 8 thatthe value of K² exceeds 8% in the range of 2.4≦KH≦4.3. Above all, in therange of 2.8≦KH≦3.8, the value of K² exceeds 8.5%.

From the symmetrical nature of LN crystallinity, the calculation isperformed only in the ranges of 0°≦φ≦30°, 0°≦θ≦180° and 0°≦ψ≦180°.Furthermore, the constants of glass used for calculation are those ofglass generally used for electronic materials. Table 2 shows theconstants of glass used for calculation.

TABLE 2 Constants of Glass Used for Calculation density 2.76 g/cm³coefficient of thermal expansion 46 × 10⁻⁷° C. dielectric constant 5.8Young's modulus 6.86 × 10³ kg/mm² Poisson's ratio 0.28

As described above, an SAW device substrate having a larger value of K²than conventional cases can be obtained by using, as piezoelectricsubstrate 2, LN in the Eulerian angle ranges which were found out to beable to attain large K² or by determining the thickness of piezoelectricsubstrate 2 and the pitch of electrodes 3 so as to be included in the KHrange which was found out to be able to attain large K². If designing isperformed so that the Eulerian angle ranges and the KH range aresatisfied simultaneously, the value of K² is made much larger and it ispreferred.

As described above, according to the surface acoustic wave device inthis embodiment, LN, which is expressed as (18-30°, 80-100°, 35-75°) interms of Eulerian angles, is used and the thickness of piezoelectricsubstrate 2 and the pitch of electrodes 3 are determined so that KH isat least 2.3 and at most 4.5. Therefore, an SAW device substrate havingthe value of K² larger than that of conventional SAW device substratescan be obtained.

Second Embodiment

A schematic sectional view showing the structure of an SAW device in asecond embodiment of the present invention is shown in FIG. 1. An SAWdevice 5 has a glass substrate 1, a piezoelectric substrate 2 and anelectrode 3. The appearance is the same as the one in the firstembodiment. In this embodiment, piezoelectric substrate 2 is formed onglass substrate 1 and is made of Li₂B₄O₇. On piezoelectric substrate 2is formed electrode 3 which has an arrangement according to the device.

Such an SAW device substrate 6 can be obtained by bonding an Li₂B₄O₇substrate as piezoelectric substrate 2 onto a surface of glass substrate1 or forming an Li₂B₄O₇ thin film as piezoelectric substrate 2 on thesurface of glass substrate 1 and, thereafter, forming and patterningelectrode 3 on the Li₂B₄O₇ substrate (thin film) as piezoelectricsubstrate 2.

Glass substrate 1 may be made of a material such as quartz glass,aluminosilicate glass, borosilicate glass, soda lime glass and leadsilicate glass. Piezoelectric substrate 2 may be made of only Li₂B₄O₇,and it may partially contain Li₂B₄O₇. The substrate to be combined withpiezoelectric substrate 2 is not limited to a glass substrate, and itmay be any material causing a lower sound speed than Li₂B₄O₇ and havingan opposite sign characteristic from those of TCD and TCV (temperaturecoefficients of sound speed) of Li₂B₄O₇. The material for electrode 3may be aluminum, for example, although not exclusive.

As shown in FIG. 1, the thickness of piezoelectric substrate 2 is H, thepitch of electrodes 3 is λ, and K is 2π/λ. The following descriptionwill be provided by paying attention to the Eulerian angles of Li₂B₄O₇included in piezoelectric substrate 2 as well as KH which is a productof K and H.

Conventionally, a <110> Li₂B₄O₇ substrate is generally used as theLi₂B₄O₇ substrate. From Table 1, the value of K² in the <110> Li₂B₄O₇substrate is 0.80%. FIG. 9 illustrates the result of calculating thevalue of KH in a (0°, θ, ψ) Li₂B₄O₇/glass structure substrate when K² is0.80%, similarly to the case of the <110> Li₂B₄O₇ substrate. For thecalculation, the Campbell and Jones method was employed. The range ofcalculated KH for Li₂B₄O₇ is 0≦KH≦3.0. The interior of the dash line isa region where K² is 0.80% while the exterior of the dash linecorresponds to an orientation in which K² is smaller than 0.80% in thecalculation range of 0≦KH≦3.0. In other words, it is where K² is atleast 0.80% in the region of 60°≦θ≦120° and 60°≦ψ≦120°.

The results of calculating the propagation velocity and TCD for thevalue of KH shown in FIG. 9, that is, for KH when K² is 0.80% areillustrated in FIGS. 10 and 11, respectively. The constants of glassused for the calculation are those typically employed for electronicmaterials. The glass constants used for the calculation are as alreadyshown in Table 2.

It can be seen from FIG. 10 that some Eulerian angles bring about alower propagation velocity although K²=0.80% as in the <110> Li₂B₄O₇substrate. Particularly for (0°, 85-95°, 85-95°), the propagationvelocity is about 2850 m/s. Since the propagation velocity on the <110>Li₂B₄O₇ substrate is 3465 m/s from Table 1, the propagation velocity onthe (0°, 85-95°, 85-95°) Li₂B₄O₇ substrate is about 18% lower than on<110> Li₂B₄O₇ substrate. Since the propagation velocity is lower, theEulerian angles (0°, 85-95°, 85-95°) can be regarded as a superiororientation. The value of KH for the Eulerian angles is about 0.5 fromFIG. 9. The value of TCD for the Eulerian angles is −15.1 from thecalculation result (see FIG. 11).

Similarly, a parameter φ of the Eulerian parameters (φ, θ, ψ) indicatingan orientation is fixed at 10° and θ, ψ are changed, and the result ofcalculating the value of KH when K² is 0.80% is illustrated in FIG. 12.Similarly, the propagation velocity and the TCD are illustrated in FIGS.13 and 14, respectively. Similarly, the cases where φ is fixed at 20°are illustrated in FIGS. 15 to 17. Similarly, the cases where φ is fixedat 30° are illustrated in FIGS. 18 to 20. Similarly, the cases where φis fixed at 40° are illustrated in FIGS. 21 to 23. Similarly, the caseswhere φ is fixed at 45° are illustrated in FIGS. 24 to 26. For each ofφ=10°, 20°, 30°, 40° and 45°, (φ, 85-95°, 85-95°) and (0, 85-95°,85-95°) have a similar propagation velocity. Therefore, by using an SAWdevice substrate provided by forming Li₂B₄O₇ having Eulerian angles of(0-45°, 85-95°, 85-95°) and particularly KH=5 on a glass substratesurface, it becomes possible to manufacture an SAW filter that issmaller than the SAW device employing the conventionally used <110>Li₂B₄O₇.

From the symmetrical property of an Li₂B₄O₇ crystal, the calculation wasperformed only in the ranges of 0°≦φ≦45°, 0°≦θ≦180° and 0°≦ψ≦180°.

An implementation of the SAW device substrate in the second embodimentof the present invention used for an SAW filter will be described. Asectional view of the SAW filter as an example of SAW device 5 using theSAW device substrate is shown in FIG. 1. A glass substrate 1 having thematerial constants shown in Table 2 and a (0°, 90°, 90°) Li₂B₄O₇substrate with a thickness of 6 μm as a piezoelectric substrate 2 arestacked by a direct bonding technique, and an electrode 3 is formed on asurface of the Li₂B₄O₇ substrate as piezoelectric substrate 2. Here, thedirect bonding technique is a technique of directly bonding thesubstrates without providing an adhesion layer therebetween.Specifically, a polished and cleaned glass substrate 1 and the Li₂B₄O₇substrate as piezoelectric substrate 2 are hydrophilized by an ammoniaaqueous solution, and thereafter the substrates are overlapped eachother and bonded together by a hydrogen bond. In this implementation,heat treatment is provided after the bonding process to increase thebonding strength. An aluminum film with a thickness of 1000 Å is formedon thus obtained Li₂B₄O₇/glass structured SAW device substrate 6 bysputtering, and the electrode is patterned by photolithography. An SAWfilter as a kind of SAW device 5 is obtained by the foregoing steps.

FIG. 27 illustrates comparison between the frequency characteristics ofSAW filters on which electrodes designed by the same parameter arerespectively formed on the <110> Li₂B₄O₇ substrate and Li₂B₄O₇/glassstructured SAW device substrate 6 in the second embodiment of thepresent invention. In FIG. 27, the frequency characteristics 7 of theSAW filter using the Li₂B₄O₇/glass substrate according to the presentinvention are indicated by the solid line, and the frequencycharacteristics 8 of the SAW filter using the <110> Li₂B₄O₇ substrateare indicated by the dash line. Here, the pitch of electrodes are set at5.2 μm to attain KH=0.5. It can be appreciated from FIG. 27 thatcomparison between the frequency characteristics indicate substantiallydifferent resonant frequencies. This is because the propagation velocityof the SAW filter according to the present invention is lower than thepropagation velocity of the conventional SAW filter using the <110>Li₂B₄O₇ substrate. In order for the SAW filter according to the presentinvention to have the same frequency band as the conventional SAWfilter, the electrode pitch needs to be made smaller. That makes itpossible to attain a smaller filter. When the characteristics in thepass bands are compared, it can be understood that the SAW filteraccording to the present invention does not exhibit any problem with theinsertion loss and the pass band width and has the value of K² similarto that of the conventional SAW filter.

As described above, according to surface acoustic wave device 5 in thisembodiment, piezoelectric substrate 2, which includes Li₂B₄O₇ expressedas (0-45°, 85-95′, 85-95°) in terms of Eulerian angles, is used aspiezoelectric substrate 2, and the thickness of piezoelectric substrate2 and the pitch of electrodes 3 are determined so that KH is about 0.5.Therefore, the propagation velocity can be made lower than that of theSAW device substrate employing the conventional <110> Li₂B₄O₇ substrate.Since the pitch of electrodes 3 can be made smaller as a result, it isadvantageous for attaining a smaller SAW device.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A substrate for a surfce acoustic wave device,comprising a glass substrate, and a piezoelectric substrate includingLiNbO₃ formed on said glass substrate, wherein said LiNbO₃ isrepresented by Eulerian angles of (24-30°, 88-92°, 55-65°).
 2. Thesubstrate for a surface acoustic wave device according to claim 1,wherein said LiNbO₃ is represented by Eulerian angles of (about 30°,about 90°, about 60°).
 3. A surface acoustic wave device, comprising asubstrate for a surface acoustic wave device having a glass substrateand a piezoelectric substrate including LiNbO₃ formed on said glasssubstrate, said LiNbO₃ being represented by Eulerian angles of (18-30°,80-100°, 35-75°), and an electrode formed on said piezoelectricsubstrate, wherein a product of K and H is at least 2.3 and at most 4.5in which λ is a pitch of said electrode, H is a thickness of saidpiezoelectric substrate, and K is 2π/λ.
 4. The surface acoustic wavedevice according to claim 3, wherein a product of K and H is at least2.8 and at most 3.8.
 5. A surface acoustic wave device, comprising asubstrate for a surface acoustic wave device having a glass substrateand a piezoelectric substrate including LiNbO₃ formed on said glasssubstrate, said LiNbO₃ being represented by Eulerian angles of (24-30°,88-92°, 55-65°), and an electrode formed on said piezoelectricsubstrate, wherein a product of K and H is at least 2.3 and at most 4.5in which λ is a pitch of said electrode, H is a thickness of saidpiezoelectric substrate, and K is 2π/λ.
 6. The surface acoustic wavedevice according to claim 5, wherein a product of K and H is at least2.8 and at most 3.8.
 7. A surface acoustic wave device, comprising asubstrate for a surface acoustic wave device having a glass substrateand a piezoelectric substrate including LiNbO₃ formed on said glasssubstrate, said LiNbO₃ being represented by Eulerian angles of (about30°, about 90°, about 60°), and an electrode formed on saidpiezoelectric substrate, wherein a product of K and H is at least 2.3and at most 4.5 in which λ is a pitch of said electrode, H is athickness of said piezoelectric substrate, and K is 2π/λ.
 8. The surfaceacoustic wave device according to claim 7, wherein a product of K and His at least 2.8 and at most 3.8.
 9. A surface acoustic wave device,comprising an electrode formed on a substrate for a surface acousticwave device having a glass substrate and a piezoelectric substrateincluding Li₂B₄O₇ formed on said glass substrate, said Li₂B₄O₇ beingrepresented by Eulerian angles of (0-45°, 85-95°, 85-95°), wherein aproduct of K and H is about 0.5 in which λ is a pitch of said electrode,H is a thickness of said piezoelectric substrate, and K is 2π/λ.